In a skillet over medium-high heat, melt the butter. Add the onion and salt and cook, stirring often, until soft and caramelized, about 20 minutes. Add the garlic and cook for 1 minute. Remove the pan from the heat. Add the Worcestershire sauce and stir to scrape up the browned bits from the pan . Place the onion in a small bowl and let cool to room temperature.
In a bowl, stir together the remaining ingredients. Add the onion. Cover and chill at least 30 minutes.